1200 HP, that's serious grunt!
As is common on S/T, you ask a simple question but get a short essay instead of an answer:
Can you increase the RR corner weight to load the tire more?
Probably easiest to just up the breakaway torque, but if not, why not go with a Detroit Locker instead of a spool? It locks up like a spool under straight line power, but ratchets to allow you to turn without chattering tires or risking a broken axle...

naylon wrote:...At around 50 mph and 60 mph one of the rear wheels spin much faster than the opposite rear wheel.

Which wheel? in a conventional RWD solid axle car, torque reaction unloads the right rear and so drag cars can have the RR weight increased to compensate. On the other hand, if yours, as it appears, is an independent rear this reaction force is resisted by the the differential mounts and the light LR weight could be causing it to spin more.